Many types of work performing apparatus include booms or arms that are pivotal about an axis so as to be readily maneuverable to a desired location. Motors are utilized to pivot the boom about such axis and in a large number of cases, the motors employed are hydraulic motors. The circuit used to control the operation of such motors is referred to as the "swing circuit" because it operates to cause the boom to swing about the aforementioned axis.
In the usual case, the swing circuit receives hydraulic fluid under pressure from a hydraulic pump. Typically, other hydraulic circuits are also powered by the same pump. Such circuits may include a circuit for controlling a grapple or a bucket, a circuit for controlling the relation of an outer boom to an inner boom in a two part boom construction, etc.
In operation of such devices, a performance drawback is in the swing circuit. Typically, many of these types of apparatus have a swing system whose operation is extremely rough and jerky. This unevenness in operation is particularly apparent during a deceleration or aiding load situation.
If one considers the apparatus boom when it is initially stationary, and hydraulic fluid under pressure is applied to the swing motor to cause boom movement, the boom will begin to accelerate as it moves to a new position. As the new position is approached, the operator will halt the flow of hydraulic fluid to the motor to halt the boom. However, because the boom has mass and is moving, it contains a sizable quantity of kinetic energy due to inertia of the moving components.
Consequently, as hydraulic fluid is shut off to the hydraulic motor, the inertia in the system causes the boom to tend to continue to swing, resulting in a so-called aiding load situation which continues until the boom is fully decelerated to a halt. This, in turn, causes the hydraulic motor to function as a pump during the deceleration procedure.
If the hydraulic fluid now being pumped by the hydraulic motor is blocked, extremely high loadings on system components result. In some systems, the torque in the swing system upon deceleration can be as much as twice the torque during acceleration. As a consequence, all of the components of the swing system must be sized for the high deceleration torque which increases both component size and cost.
To avoid these problems, a variety of systems have been devised. In a very simple system, torque reliefs are located in the control valves to provide hydraulic braking as well as to protect the hydraulic motor during deceleration.
Most of these systems have met with some success. However, because swing systems are bi-rotational, that is, used to rotate a boom or the like in both directions around a pivot point, acceleration and deceleration pressures must be the same, and this necessitates "over building" to accommodate the high deceleration torque.
To overcome this difficulty, it has been proposed to use two stage dual level cross-over relief valves. This arrangement employs pilot operated pressure relief valves that are cross-connected across the swing motor. These valves are such that when they receive a pilot signal, they will open as pressure relief valves only at a relatively high pressure, whereas when no pilot signal is present, they will operate and open as pressure relief valves at a relatively low pressure.
These systems require a controller to select the pilot pressure to achieve the desired result and as a consequence, the expense of providing a controller is not conducive to a number of types of smaller, less expensive apparatus of the type having swing circuits.
Still other systems utilizing cross-connected two stage dual cross-over relief valves may utilize a manually operated control valve to control pilot pressure. For example, a foot operated valve might be utilized. While such systems are operative, they have the disadvantage of requiring additional hydraulic lines as well as excessive manual control effort.
In my above identified co-pending application, I have proposed a hydraulic swing circuit that is of a simple configuration and which provides an economical solution to the problems identified above. In one embodiment of the invention of my co-pending application, a pair of hydraulic conduits are provided to extend from the control valve to the hydraulic motor of the swing circuit. A pair of pilot operated relief valves are connected oppositely across the conduits and a pair of pilot operated check valves are provided, one in each of the conduits, at a location between the control valve and the relief valves. The pilots of the relief valves are connected to the conduit whose pressure is relieved by the associated relief valve at a location between the relief valve and the control valve while the pilot of each check valve is connected to the conduit in which the other check valve is located at a location between the check valve and the control valve. Each check valve is oriented in the associated conduit to allow flow from the control valve toward the hydraulic motor, but not the reverse, except when receiving a pilot signal.
As a consequence of this construction, upon deceleration, when the motor begins to act as a pump rather than a motor, the resultant lowering of pressure allows one of the relief valves to recirculate fluid back to the opposite side of the motor to provide fluid at low pressure levels for smooth deceleration.
Such a hydraulic swing circuit works well in the vast majority of instances. However, in one situation, there remains the possibility of a somewhat uneven operation.
Those skilled in the art will readily recognize that swing circuits of the type of concern are frequently employed with work performing devices such as loaders or back hoes that are mounted on vehicles. In most instances, outriggers are employed to maintain the vehicle in a level and stable position when the swing circuit is being operated. If, however, the base on which the swing circuit is mounted is not level, when the swing circuit is moving the work performing implement in the "down hill" direction, an aiding load situation occurs. That is to say, the weight of the work performing instrument and any load carried thereby, acts in addition to the hydraulic swinging force applied. As pumping action resulting from the aiding load increases, the check valve closes due to a drop in pressure at it's pilot, stopping the pumping action. Hydraulic fluid under pressure continues to be supplied via the control valve with the consequence that pressure in the opposite line increases causing the pilot to again open the check valve. Again, the aiding load causes the pump to act as a motor, driving hydraulic fluid through the check valve and causing a lowering in pressure at the pilot for the check valve. The resulting drop in pressure causes the check valve to close.
As can be readily appreciated, the situation is prone to cycling, causing uneven movement of the work performing device.
The present invention is directed to overcoming one or more of the above problems.